Brian:
To cut to the chase, just tell them to reverse the red/green. Instead of green/green and red/red at their jack, just tell them to make it green/red and red/green.
This isn't your fault. There is nothing wrong with the phones. The customer's jacks are just wired improperly.
I'm sure that you have no intention of receiving the phones back and installing polarity guards in order to complete the sale. That would be silly.
